I'm not sure Bradley Cooper would have been a great GL/much better than Reynolds but he probably would have been better. Cooper especially, though similarly relatable, has/could have had a lot of charisma. With Reynolds as he was portrayed it seemed like the studio was uncertainly hedging a lot, trying to cram in a lot of self-humor for if it wasn't otherwise impressive.
I also think the special effects, including the Green Lantern suits, actually were and, from the marketing, also looked pretty bad/generic. And the plot was weird and hard to describe or get people excited about through the marketing.

__________________"It's funny, isn't it? A life of truth, justice and the American, military, capitalist way. In the end, you die on the moon, wondering if your bowels will let go. And tomorrow, the world will go on like nothing happened. Dragged kicking and screaming to a better day." Manchester Black - Superman vs The Elite